It is known to prolong and safeguard the shelf life of perishable products by modifying the atmosphere in the free space remaining in the package after filling the product. The most common gases used in a modified atmosphere package (MAP) are nitrogen, CO2 and oxygen or mixtures thereof. Oxygen is mainly used for keeping fresh meat red or for controlling the ripening rate of fruit and vegetables, whereas nitrogen and CO2 is used for reducing the ageing effects of oxygen and reducing aerobic growth of micro-organisms in a multitude of products.
The packaging material in MAPs can be any essentially gas tight material including plastic film or coating, film coated paper and thin metal foils.
A leak in the package due to production errors or from later damage will result in at least a partial loss of the modified atmosphere and the predicted shelf life may be shortened.
A common cause of leakage is product residue or splashes deposited on the sealing surfaces prior to closure with welding or other techniques. Studies of the effect of different leak diameters therefore assume a length of the leak path in the order of several millimeters.
Maximum tolerable leak diameters in the food industry range from 5 to 150 μm depending on the content and the purpose of the package.
For MAPs intended to prevent microbial growth and oxidation of sterile and antiseptic content maximum the leak diameter is typically observed to be in the range of 5-25 μm whereas tolerable leaks for ready-to-eat meals with shorter shelf life is in the range of 30-50 μm.
Baked and dried goods mainly susceptible to mould growth and humidity changes can normally tolerate leak diameter of 50 to 130 μm.
Various leak testing arrangements have been presented and, in some cases, also used to prevent leaking packages from reaching the shop shelves.
A common limitation in many known methods is that they cannot perform 100% testing for the quoted leak diameters on individual packages at full production line speeds exceeding 30 packages per minute. This problem can be circumvented by either restricting testing to off-line spot testing or by making batch testing of a suitable number of packages in the same test cycle.
One possible way to increase testing capacity is to provide a number of parallel testing arrangements, each working at a lower speed but collectively matching the cycle time of the filling line.
EP 0 894 252 B1 discloses a leak testing method allegedly capable of performing individual package testing “very quickly, typically in about one second” indicating testing capacity of up to 60 packages per minute.
The method disclosed in EP 0 894 252 B1 employs diluted hydrogen as tracer gas. Hydrogen and helium are commonly used as tracer gases for leak testing. One problem with these gases is their relatively high diffusion rate through the thin polymer films and other packaging materials used as gas barriers in packaging materials. If the tracer gas diffuses through the non-leaking walls of the package there is a risk that the leak testing arrangement will detect a diffusion leakage that is not a package integrity problem. This problem can be overcome if the time between filling the gas and testing for leakage is sufficiently short so that the leak detection is performed before the gas has permeated through the barrier material. This break-through time is in the order of a few seconds to several minutes depending on the material used and on the thickness of the film.
The method disclosed in EP 0 894 252 B1 has in practice not been demonstrated to fulfil the production speeds promised. The mains reasons for this are:                The response time of the proposed type of sensor (palladium film) is too long for high speed leak testing. This is in fact true for almost all commercially available hydrogen sensors.        The recovery time of the proposed sensor is too long. The recovery time is in the order of 1-10 seconds which holds true for most commercially available sensors.        After detecting a larger leak there is a period during which the sensor exhibits a reduced sensitivity, which means that there is a period of typically 15-90 seconds after the detection of a large leak during which the sensor cannot detect smaller leaks.        
The response time of a sensor is defined as the time needed for the sensor to reach 90% of the final signal value when exposed to a certain concentration of gas. Typical response times for most commercial hydrogen sensors are in the order of 5-30 s. Even with response time reducing algorithms, the response time achieved by commercial sensors are never below one second for the concentration values typically used in EP08942528.
Reaction time, which is the time to reach 10% of full signal, is often quoted and is often erroneously called “response time”. The reaction time can be down to 0.1 seconds for the best commercially available sensors.
Both reaction and response times are typically longer for lower concentrations. This is due to diffusion processes inside the sensor housing and inside the active material of the sensor. The rate of concentration increase of gas inside the sensor housing and inside the sensing material is slower the lower the concentration gradient is in front of the sensor which results in slower reaction and response for low concentrations.
One way of reducing the reaction and response times is to operate the sensor at a higher temperature. This enhances diffusion in the housing as the gas in heated and the diffusion inside the sensing material is likewise enhanced, whereby response and recovery time is reduced. Such sensors are, however, always mounted in bigger housings to reduce loss of thermal energy and to ensure that materials used are not destroyed. Most of the improvements in response time are therefore lost on a system level and it remains problematic to reach the production rates wanted within the industry.
Another way of reducing the reaction and response time is to minimise the distance the gas needs to diffuse from the flow path outside the sensor housing in to the surface of the actual sensing material. The diffusion inside the sensing material also plays a major role for the response time.
EP 0 894 252 B1 employs a chamber in which the package is placed. The chamber is subsequently closed and a limited negative pressure is drawn.
The document argues that this limited vacuum level, as compared to levels used in previous arrangements for gas testing, can be achieved quickly and with low cost means. The packages should be kept in the chamber for a dwell time claimed to be in the range of 0.5 to 60 seconds.
EP 0 894 252 B1 does not clearly explain the different steps contributing to the total cycle time. The total cycle comprises at least the following steps:    1) Loading of a package. Moving the package into position and closing the chamber.    2) Chamber pressure reduction.    3) Sampling time. The time needed to bring the tracer gas from the exit of the leak to a position directly in front of the sensor.    4) Reaction time of the sensor.    5) Sensor response time. Time needed by sensor algorithm or circuitry to decide level of leakage. This is the time during which the signal is recorded or tracked to give sufficient information for a fair estimate of the magnitude of leakage.    6) Chamber flushing and sensor recovery in case of a leak signal.    7) Unloading the package.
The dwell time mentioned in EP 0 894 252 B1 would then seem to include the steps 3 to 5.
It is possible to partially reduce the time needed for some of these steps by letting these steps be carried out in parallel. It is for example evident that loading of the next package can be carried out in the same sequence as unloading the just tested package. Another example of such optimization is to allow for some of the sensor recovery time take place during the unloading/loading step.
However, even with the best practice, additional time is needed for decreasing the pressure in the chamber and, most of all, for true sensor recovery. The sensor recovery time is long and often not entirely predictable.
This poses large restraints on the automation system in that the process cannot run at a constant speed.
The invention disclosed in EP 0 894 252 B1 can therefore not fulfil production speeds faster than 60 packages per minute that is requested in industry today. It is even doubtful if a speed of 30 packages per second can be reached.
This means that there is still a need for a robust, fast and reliable leak testing method and arrangement for packages containing perishable products in the form of cosmetic, chemical or pharmaceutical products, toothpaste, a foodstuff or similar. Similar needs for high speed testing also exists in other areas such as for example within the refrigeration and automotive industries.